1. Field of the Invention
The present invention relates to an image reading apparatus and multifunction printing apparatus and, particularly, to an image reading apparatus and multifunction printing apparatus which optically read an image original by using a white LED.
2. Description of the Related Art
There has conventionally been known a scanner apparatus which optically reads an image.
FIG. 9 is a side sectional view showing the arrangement of the optical unit of a conventional scanner apparatus. When reading an image, light which has been emitted by an LED 15 serving as a light source and passed through a light guide 1 irradiates an original image through a platen glass 8, as shown in FIG. 9. The light reflected by the original image forms an image through an imaging optical system formed from reflecting mirrors 2, 3, 4, and 5 and a condenser lens 6. The image forming light is converted into an electrical signal by using a CCD 7.
In early scanner apparatuses, one beam was separated into three components by using a prism for color separation when reading a color image. Thus, the R, G, and B color components were read at one portion. However, such a scanner apparatus using the prism required three or more CCDs, and the number of components increased along with this. The CCD positions for reading the R, G, and B color components needed to be adjusted, and it was very difficult to adjust them. Further, the prism was expensive and raised the cost.
To solve these problems, a scanner apparatus using a 3-line CCD has been proposed. However, in the scanner apparatus using the 3-line CCD, the reading positions of the R, G, and B color components shift in the sub-scanning direction.
FIG. 10 is a view schematically showing dispersion of light in a scanner apparatus using a 3-line CCD. As shown in FIG. 10, reading positions for light emitted by the LED 15 through the light guide 1 shift in correspondence with the R, G, and B color components in the sub-scanning direction on an image original (not shown) placed on the platen glass 8.
Recently, thanks to technological development, the LED luminous efficiency has been improved close to the luminous efficiency of a CCFL (Cold Cathode Fluorescent Lamp) conventionally used as the light source of the scanner apparatus. This light source is being replaced with a mercury-free LED. For example, Japanese Patent Laid-Open No. 1-181377 discloses an LED illumination device which separately illuminates the reading positions of three lines in order to cope with a 3-line color CCD. Japanese Patent No. 3,217,879 discloses a method of obtaining a uniform illumination light amount as a whole by condensing light from a columnar lamp to different locations in the vertical direction on a reflection board. Further, Japanese Patent Laid-Open No. 2008-123766 discloses the shape of a light guide which efficiently guides light from a white LED onto a platen glass surface.
In a case where the 3-line CCD is used, the R, G, and B reading positions shift in the sub-scanning direction, as described above, so the illumination device needs to uniformly irradiate the reading width. For example, FIG. 10 shows the width as a CCD line width. If this width cannot be illuminated uniformly, reading outputs from the respective lines of the CCD vary, and accumulated charges become insufficient. As a result, noise increases or read information becomes incorrect, and the read color changes from the original color.
The emission direction of the conventional scanner apparatus using the CCFL is 360°, the light is not directional, and the light amount distribution width and the like can be adjusted using the reflection board. However, when the light source is replaced with the LED to also stop the use of mercury, LED emission has high directionality. That is, not the entire light source emits light, but only one small emission surface substantially emits light of a spot shape, unlike the CCFL. It is therefore difficult to adjust the light amount distribution by using the reflection board.
To solve this, in a case where the LED 15 is used as the light source, as shown in FIG. 10, the emission direction and diffused direction of light are controlled using the light guide 1 to obtain a satisfactorily uniform light amount distribution on the platen glass 8. FIG. 10 schematically shows the distribution of the amount of light from a surface 9 of the light guide 1. FIG. 10 shows the result of a ray tracing simulation in which light emitted by the LED 15 propagates through the light guide 1 while being totally reflected, and reaches the upper surface of the platen glass 8.
Recent scanner apparatuses are increasing the resolution, and CCD pixels need to be arranged in the main scanning direction in accordance with the resolution. However, the integration technology has limitations. To cope with a high resolution while keeping the optical unit compact, CCD pixels need to be staggered and arrayed on a plurality of lines in the sub-scanning direction. Hence, the distribution of light needs to uniformly illuminate the respective CCD lines spread in the sub-scanning direction, and read the light.
In short, in a case where the CCD line width is sufficiently small, a uniform light amount distribution is obtained on the platen glass surface by light from the light guide. However, in a case where the CCD line width is large in the sub-scanning direction, the light amount becomes greatly different between the respective color components, as represented by B2, G1, and R1 shown in FIG. 10.
As a countermeasure against this, the arrangement is returned so that one beam is separated into three color components and the R, G, and B color components are read at one portion. In this case, a change of the tint can be suppressed though the image becomes dark. However, this poses the problems of cost and productivity, as described above.
As shown in FIG. 9, light from the light source of a scanner apparatus generally irradiates an image original from only the diagonal direction, while the reading light path of the CCD is vertical. Because of this arrangement, it is very difficult to prevent changes of the light amount and tint in a reading range corresponding to a large CCD line width. For this reason, a method is conceivable, in which the CCFL is used, and positions at different levels are illuminated with two illumination units, as disclosed in Japanese Patent No. 3,217,879. More specifically, two LED boards on each of which about 30 compact LEDs are arranged side by side, and two light guides are provided, and are arranged on one side of the reading unit. This arrangement can be expected to obtain the same effects as those described above. Further, these illumination units are arranged on the two sides of the reading unit and illuminate positions at the same level. This makes at least a change of the light amount distribution bilaterally symmetrical. The output balance between R and B is well-maintained, a change of the tint is greatly reduced, and a uniform illumination effect can also be expected.
In this arrangement, since the light guide is a simple plastic component, even if a plurality of such light guides are used, they hardly influence the cost. However, LEDs and a board on which the LEDs are arrayed are very expensive. In terms of cost, it is hard to employ the arrangement of a plurality of LED boards for a product.